If I had to speculate, I would say the brown shading is a heat reaction rather than a layer thickness thing because the yellow glass at the base is thicker than the brown part (there is a teeny very shallow nick in the brown that goes through to the white).

It has two layers of uranium glass: yellowish brown over whitish. The gilding is good quality and all raised (and not over yellow like Bohemian tends to be). It has the "Webb butterfly". The flowers and buds look just like those on Webb vases on p25 From Palace to Parlour.

The base has a polished pontil mark and a polished rim. The vase it quite heavy for its size.